The continuous casting system provides a system of casting fabrication in which a supply of molten metal or metal alloy is heated and liquified within a furnace-like structure called a tundish or heated outside the tundish and placed therein prior to casting. In most systems the furnace includes a discharge orifice near the bottom of its internal cavity which is coupled by a throat to a cooled die or mold. The latter defines an elongated die passage suitable for the formation of an elongated casting which in turn defines an entrance opening and an exit opening. In addition, cooling means are provided which generally encircle or surround the die passage for the purpose of conducting sufficient heat from the molten metal within the die passage to solidify all or part of the molten metal therein and form the casting. Continuous casting systems may comprise either vertical or horizontal casters.
Vertical casting systems are generally used to form large billet and slab castings and acquire their name from the vertical casting path. The furnace and cooled mold are arranged vertically and gravity flows the molten metal into and through the mold. In most vertical casting systems, an array of drive rollers beneath the mold control the downward motion of the casting. In many vertical casting systems a gradual curve is introduced into the casting to transition it from a vertical path to a horizontal path in order to reduce the overall height of the casting system.
In horizontal continuous casting systems, the furnace, called a tundish, and the cooled die, also called a mold, are horizontally aligned and drive means are provided downstream of the mold which are operative upon the casting to periodically withdraw a portion of the casting from the die passage. The speed at which the casting is withdrawn from the cooled die is selected in accordance with the cooling capacity of the die and characteristics of the casting to ensure that the emerging casting is solidified on its outer surfaces to a sufficient extent that the forces imparted by the drive system do not cause the casting to be overstressed and damaged.
In both horizontal and vertical casting systems, the casting of thicker casting configurations results in withdrawing the casting before complete solidification has taken place in the mold. As a result, the casting emerging from the cooled die passage has a solidified outer skin with a molten center. The molten center is generally tapered from a maximum cross-section near the casting's emergence from the cooled mold to a minimum at the point of complete solidification of the casting. The distance from the input orifice of the cooled mold to the point of complete solidification of the casting is known as the "metallurgical length". For reasons which are well-known in the art, the casting quality is improved as the metallurgical length is shortened. That is to say, with shorter metallurgical length and the faster cooling which produces it, the average grain size within the casting is finer, which is the desired characteristic. In addition, a shorter metallurgical length minimizes the formation of internal voids and permits the rolling stages of the casting system to be located closer to the mold thereby reducing the length of the casting system. In addition to the need to cool the casting which arise in attempts to reduce metallurgical length, another problem arises because of great heat present in the molten center. The casting skin must be cooled after the casting emerges from the cooled die to prevent the casting skin from being melted by the heat present in the molten metal within the casting. This problem, known as "remelting", is avoided by utilizing either or both of two basic cooling systems. The first, uses a long cooled die or mold having sufficient capacity to withdraw substantially more heat from the casting than is required to form the above-described skin. The use of a long casting mold or cooled die provides some additional cooling of the casting. However, a problem arises in both vertical and horizontal continuous casting process caused by shrinkage of the casting as cooling takes place. This shrinkage tends to distribute itself down the casting and result in a reduced cross-sectional area and surface area of the casting as a function of distance from the tundish. In essence, the casting assumes a "tapered shape". In most castings, the casting taper is sufficient to cause an air space to be created between the casting skin and the cooling surfaces of the cooled die passage as the casting "shrinks" away from the passage walls. Once the contact between the passage walls and the casting surface is broken, the cooling of that area of the casting is decreased reducing overall cooling and creating "hot spots" in the casting. In addition, because some portions of the casting remain in contact with the die passage and are cooled more rapidly than those no longer in contact, uneven cooling results which degrades casting quality and often causes the casting to warp. Practitioners in the art have attempted to compensate for casting shrinkage by simply constructing the die passage to include a carefully designed taper which gradually narrows the die passage as a function of distance from the entrance orifice or tundish.
The use of tapered die passages within the mold structures provides some improvement in the ability of the cooled die to compensate for the shrinkage of the casting. However, because each casting configuration and size and each metal or metal alloy used requires a different shrinkage taper, the mold or cooled die taper must be customized for each application. This leads to increased fabrication and tooling costs which are prohibitive in a competitive environment. In addition, for each casting and metal or metal alloy cast, the passage taper is fitted to a casting stroke, speed and superheat. Therefore, the casting stroke and speed must be inordinately controlled. Further, tapered molds or dies are less tolerant of wear due to the precision required of the taper.
The second approach utilizes one or more casting cooling devices known as secondary spray cooling zones located in the downstream portion of the casting path near its emergence from the cooled die which are operative to withdraw further heat from the casting. In the majority of the present systems, such secondary spary coolers comprise a plurality of water spray devices which direct water streams or air and water mist at the emerging casting intended to carry heat from the casting surface.
Generally, such secondary spray coolers are only partially effective however, and often produce large quantities of steam which require collection and are sensitive and difficult to maintain. As a result, many practitioners in the casting art have been forced to use longer casting dies and live with the difficulties and increased costs associated with extended cooling dies and water spray coolers. Other practitioners have attempted to construct aftercoolers having greater effectiveness than the conventional spray coolers heretofore used in the hope of avoiding the need for spray coolers. Prior attempts at improving aftercooler effectiveness include the provision of aftercoolers which are in essence similar to the cooled die which originally formed the casting. As such, these aftercoolers must compensate for the shrinkage and are therefore tapered to match the inherent taper of the cooling casting. Recognizing the difficulties and limitations of tapered passage aftercoolers, other practitioners in the art have attempted to provide aftercoolers having walls which are moveable to accommodate the variations in casting taper and thus avoid the expenses and difficulties of custom designed tapered equipment for each application.
Prior attempts at providing aftercoolers having wall structures which accomodate a variety of casting tapers have resulted in structures which are only partial solutions in that they contact only portions of the casting surface. Such systems, as shown and described in U.S. Pat. No. 3,580,327, U.S. Pat. No. 4,308,774, and U.S. Pat. No. 3,467,168, provide structures which contact only portions of the casting surface. While such structures provide an improvement in aftercooler design, they do not provide a casting encompassing passage way which automatically interracts with the casting so as to contact the entire casting surface including its corners. As is well understood by those skilled in the casting art, complete contact with the entire casting surface including its corners is essential to the attainment of even cooling of the entire casting in order to provide the desired casting uniformity and grain structure as well as prevention of the remelt phenomenon.
In addition to problems associated with the taper of the casting, all molds and aftercoolers, regardless of design, are subject to substantial wear as the heated casting is moved through the structure. In the case of fixed tapered molds in particular, such wear quickly renders the taper inappropriate for proper cooling of the casting. To a lesser extent but still nonetheless significant, cooling structures utilized as aftercoolers in which some of the cooling walls are moveable often result in unequal wear between the moveable and fixed walls. This of course produces a corresponding deterioration in the ability of the device to accommodate casting taper.
The problem of constructing aftercoolers is further exaserbated by the structure of the cooler walls themselves. In the majority of such aftercooler devices, the walls are multi-layered combinations of elements. Each includes an interior surface selected to provide reduced friction, such as graphite, and a backing plate selected for its strength and heat transfer capabilities, such as copper, together with an outer plate generally comprising a rigid steel mounting plate selected for strength and rigidity. One or more coolant passages for circulating a liquid coolant are formed in the copper backing plate and the steel mounting plate.
While the above-described prior art structures have provided some improvement in casting cooling and a partial solution to the problem of accommodating casting tapers, they have not as yet provided aftercooler structures in which the casting taper is accommodated in a manner whereby the aftercooler maintains contact with the entire surface of the casting including its corners. There remains therefore, a need in the art for an improved aftercooler for use in continuous casting systems which maintains contact with the entire surface of the emerging casting and which accommodates the varying tapers of the cooling casting while maintaining surface contact.